Method of manufacturing composite panels

ABSTRACT

A method of manufacturing solid composite panels utilizes an exothermic foam-forming resin to simultaneously consolidate and cure the heat activated resin of the composite panel without forming a foam-filled part. According to the method, composite fabric layers are laid up on the interior surface of a mold half that conforms to the surface configuration of the finished panel. Once the composite fabric layers are laid up, a flexible bladder is placed over the composite layup and the mold closed. The exothermic foam-forming resin is injected into the bladder within the mold cavity, which generates heat to cure the resin impregnated fabric layers above the cure temperature while simultaneously generating pressure within the cavity to consolidate the fabric layers into a substantially solid void-free composite panel.

BACKGROUND OF THE INVENTION

The present invention relates to the fabrication of composite panels, more particularly to the manufacture of composite panels for use in medical equipment and residential construction.

In U.S. Pat. No. 6,117,376 (the “376 patent”) the inventor of the present invention describes a method of making foam-filled composite products, in which fiber layers, impregnated with a heat activated resin are built up on two mold halves, which are then clamped together to form a pressure vessel. An exothermic foam-forming resin is then poured into the mold cavity. The exothermic foaming reaction of the foam material generates both heat and pressure to consolidate and cure the resin impregnated fiber layers while forming a tight bond between the foam and the fiber layers. Advantages of the method described in the '376 patent includes substantial savings in production costs by obviating the need for expensive autoclaves, vacuum furnaces, heated molds or other capital-intensive equipment.

There are, however, applications where a solid composite panel, rather than a foam-filled composite structure, may be advantageously used. Unfortunately, to manufacture thin composite panels prior to the present invention, one had to resort to the capital equipment-intensive methods such as vacuum furnaces and autoclaves. What is needed, therefore, is a method of making solid composite panels utilizing a self-heating and pressurizing method similar to that taught in the '376 patent.

SUMMARY OF THE INVENTION

The present invention comprises a method of manufacturing solid composite panels utilizing an exothermic foam-forming resin to simultaneously consolidate and cure the heat activated resin of the composite panel without forming a foam-filled part. According to an illustrative embodiment of the present invention, composite fabric layers are laid up on the interior surface of a mold half that conforms to the surface configuration of the finished panel. The composite fabric layers may be so-called “pre-preg” sheets, which comprise fabric that has been pre-coated with resin, or they may be dry sheets that are wetted with resin after they have been placed in the mold half. Once the composite fabric layers are laid up, a flexible bladder is placed over the composite layup and the mold closed by means of a rigid cover plate so as to form a cavity inside the mold. A foam-forming resin, is pre-measured so as to expand to more than fill the cavity. The foam-forming resin is injected into the bladder within the mold cavity. The mix begins foaming after a short delay, generating an exothermic reaction that heats the resin impregnated fabric layers above the cure temperature. Simultaneously, the expanding foam generates pressure within the cavity. The heat generated by the exothermic reaction initiates and maintains curing of the wet resin because the thermal energy is largely retained within the mold. The internal pressurization simultaneously consolidates the fabric layers into a substantially solid void-free composite panel. A post-curing interval can be employed to assure full stabilization of the composite product. Thereafter, the mold may be opened and the product released from the mold, and the foam-filled bladder discarded.

Because the mold half and the cover plate have strong pressure resistant outer walls, but only the interior portion of the mold half needs to have a precise, part-forming shape, the outer wall of the mold half and the cover plate can be reused for a variety of similar applications. Virtually no capital equipment is required for the manufacture of the molds or parts in small or large quantities. No external power is required to generate heat or pressure. In consequence, a wide variety of composite parts heretofore requiring massive capital investment for autoclaves, vacuum furnaces and the like can be manufactured on site with minimal equipment and investment.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawings figures in which like references designate like elements and, in which:

FIG. 1 is an exploded perspective view of a mold for manufacturing a composite panel incorporating features of the present invention;

FIG. 2 is a partial cross-sectional view of the mold of FIG. 1 in a closed condition;

FIG. 3 is a perspective view of a composite panel manufactured in accordance with the teachings of the present invention;

FIG. 4 is an enlarged cross-sectional view of a composite panel manufactured in accordance with the teachings of the present invention;

FIG. 5 is a partial cross-sectional view of an alternative embodiment of a mold for manufacturing composite products incorporating features of the present invention; and

FIG. 6 is an exploded perspective view of an alternative embodiment of a mold for manufacturing composite panels incorporating features of the present invention.

DETAILED DESCRIPTION

The drawing figures are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and the detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention.

With reference to FIGS. 1-3, medical devices such as orthopedic leg braces must be strong and durable in order to provide the necessary protection for the wearer. At the same time, such devices must be made of lightweight materials to provide maximum comfort. Accordingly, lightweight, high-strength composites such as graphite-epoxy are increasingly being used in the main structural components of these devices. Generally, orthopedic devices are custom-fitted to the patient. Since the doctor's office does not have the manufacturing facilities on-site, typically the doctor or the doctor's assistant must take the appropriate measurements, which are transmitted offsite to the fabricator. The fabricator then molds the parts or selects pre-fabricated components based on the measurements and assembles them into a device having the appropriate dimensions. Because the doctor's office and the fabricator are often hundreds or thousands of miles away, turnaround time is slow and, since prior art fabrication techniques are used to mold the components, the resulting product is unnecessarily bulky and/or costly.

In accordance with the present invention, a composite panel such as leg brace 10 can be fabricated on-site at the doctor's office with a minimum investment in capital-equipment by a technician with minimal training. A mold assembly 13 consisting of a lower mold half 12 and an upper mold half such as mold cover 14 comprise the only capital equipment necessary to carry out the molding process. Preferably lower mold half 12 and mold cover 14 are themselves made of carbon fiber composite, ceramic, ceramic-lined metal or other durable material having a low thermal conductivity. Inner surface 16 of lower mold half 12 comprises a female mold portion 18 that conforms to the configuration of outer surface 20 of leg brace 10. Preferably female mold portion 18 comprises a silicone line 22 placed within lower mold half 12. Silicone liner 22 has an outer surface that matches the recess in lower mold half 12 and an inner surface that is cast to conform to the custom dimension of the leg brace being fabricated. Use of replaceable silicone liners within mold half 12 permits lower mold half 12 to be utilized for fabricating a wide variety of shapes and sizes, thus permitting leg brace 10 to be customized to each individual patient.

A first fabric layer 24 is placed inside lower mold half 12 within the recess defined by female mold portion 18 of silicone liner 22. First fabric layer 24 may be made from any of a wide variety of structural fabric materials such as carbon fiber, aramid fiber, glass fiber and ceramic fiber is either dry or “pre-preg” (pre-impregnated with resin) sheets. In the illustrative embodiment, first fabric layer 24 comprises a pre-preg sheet of woven bi-directional 0.006 inch bi-directional carbon fiber, coated with a B-Staged (i.e., heat activated) epoxy resin sold by Cass Polymers, of Charlotte Mich. under the trade mark PROBUILD, however other fibrous materials and resin systems such as 0.006 inch uni-directional carbon fiber, glass fiber, aramid fiber, ceramic fiber or other flexible fabric materials coated with heat-activated resins may be used without departing from the scope of the present invention.

One or more immediate fabric layers such as fabric layer 26 are positioned atop first fabric layer 24 within lower mold half 12. Fabric layer 26 also may comprise any of a wide variety of structural fabric materials such as graphite fiber, aramid fiber, glass fiber, or ceramic fiber, but in the illustrative embodiment comprises a pre-preg sheet of uni-directional graphite material such as C.F. 7440-44-0 made by Hexcel Composites of Stamford Conn., coated with a B-Staged epoxy such as the aforementioned PROBUILD epoxy resin. Depending on the application, additional layers of uni- and bi-directional structural fabric may be added to achieve the strength and rigidity needed for the particular application. Once the proper number of layers have been built up within lower mold half 12, a mold release layer 28 is positioned over the structural fabric layers 24 and 26. Depending on the desired finish, mold release layer 28 may be any number of film or woven sheets designed for such purposes such as mylar, which yields a glossy surface or polytetrafluoroethylene mold release sheets manufactured by Precision Coatings, Inc. of Walled Lake, Mich. Mold release layer 28 freely releases from the resins that are cured in direct contact with its surface while having the additional advantage of avoiding contamination of the resin surface with release agents such as wax based materials.

A bladder 30 is then place atop mold release layer 28. In the illustrative embodiment, bladder 30 comprises a polyethylene bag having a volume substantially in excess of the cavity defined by inner surface 16 of lower mold half 12. Bladder 30 is then filled with a pre-measured portion of an exothermic foam-forming resin 34 such 20-10 foam resin manufactured by Polytech Foam Products, Inc. a division of FAI International, of Richmond Calif. Typical foam resins expand to form a foam having a density from 2 pounds per cubic foot to 24 pounds per cubic foot and can be selected based on the desired pressure and heat to be exerted on the fabric layers. Before the foam material expands significantly, mold cover 14 is attached to mold lower half 12 by means of screws, clamps or otherwise such that inner surface 16 of lower mold half 12 forms a substantially sealed interior volume 32.

The foam-forming resin 34 begins to expand rapidly and builds up interior pressure within interior volume 32 thus consolidating fabric layers 24-26 into a solid void-free body. The reaction of the foam-forming resin 34 is also strongly exothermic and heats the foam to an excess of 300° F. as it expands. The heat from the foam-forming resin 34 is conducted easily through bladder 30 into the resin surrounding the fabric layers 24-26. The insulating properties of lower mold half 12 and mold cover 14 retain the heat within the interior volume 32 such that the temperature in the resin surrounding fabric layers 24-26 remains above the heat activation/curing temperature for a pre-determined period of time long enough to ensure full curing of the resin.

Once the resin surrounding fabric layers 24-26 is fully cured, mold cover 14 is removed and the cured part consisting of fabric layers 24-26 is removed from lower mold half 12. Bladder 30 containing the now-cured foam-forming resin 34 is then separated from cured leg brace 10 and is discarded, as is mold release layer 28. The final part is then used as-cast or is finished with paint, gel coat or other surface finish.

With reference to FIG. 4, in order to increase the part thickness without adding additional often costly layers of carbon fabric, an intermediate layer 36 may be added between various layers of fabric layer 26. In one embodiment, intermediate layer 36 comprises an exothermic non-foaming fast-setting urethane polymer, which acts as a spacer between fabric layers 26 to increase the cross sectional moment of inertia thereof. The exothermic nature of the urethane intermediate layer 36 also assists in elevating the temperature of the resin surround fabric layers 24-26 to above their cure temperatures. It is not necessary, however, that intermediately layer 36 be an exothermic urethane resin. Other filler materials including fiberglass cloth or other cloth materials less costly than carbon fiber may be used as intermediate layer 36.

Since exothermic foam-forming resin 34 assumes whatever irregular shape is present within interior volume 32 of lower mold half 12, it is possible to use the exothermic foam reaction to consolidate and heat multiple parts at once. With reference to FIG. 5, a pair of leg braces 40-42 may be cured simultaneously by laying up leg brace 40 in lower mold half 12 than laying up a second leg brace 42 in a mold cover 38 that is identically contoured to lower mold half 12. Once the mold is closed and the exothermic foam-forming resin 34 is injected into bladder 30 contained within lower mold half 12 and mold cover 38 the exothermic foam-forming resin 34 simultaneously heats the resin surrounding the fabric layers comprising leg brace 40 and 42 while simultaneously heating the resin to above the cure temperature until the resin is fully cured. Once the mold is opened and bladder 10 containing the cured foam-forming resin 34 is separated and discarded, leg brace 40 is separated from lower mold half 12 and a second leg brace 42 is removed from mold cover 38.

With reference to FIG. 6, implicit in the discussion of FIGS. 1-5, an advantage of using an exothermic foam-forming resin to pressurize the interior volume 32 of the mold is that the pressure generated by the foam is omni-directional and, therefore, consolidates the fabric layers irrespective of their orientation. A process in accordance with the present invention can, however, equally apply to a relatively flat panel such as a door skin 50 as shown in FIG. 6. As with the illustrative embodiment of FIGS. 1-5, the process begins with providing a lower mold half 54, the interior surface of which defines the final surface configuration of door skin 50. Because the surface finish of door skin 50 is critical, however, a gel coat such as ES 201 PC Surface Coat manufactured by Cass Polymers, of Charlotte Mich. is applied to the interior surface 56 of lower mold half 54. Since the gel coat 62 undergoes a catalyzed exothermic reaction, the gel coat 62 also assists in the heating of the interior volume 58 of mold assembly 60. Once gel coat 62 has been applied to interior surface 56 of lower mold half 54, a first fabric layer 52 preferably a plurality of fabric layers are laid-up in lower mold half 54. Thereafter bladder 64 containing a pre-measured quantity of exothermic foam-forming resin 66 is placed inside interior volume 58 of lower mold half 54. Thereafter, mold cover 68 is placed over lower mold half 54 and clamped or bolted together. As with the illustrative embodiment of FIGS. 1-5, a mold release layer optionally may be used to prevent bladder 64 for adhering to door skin 50 during the curing operation. Also, as with the illustrative embodiment of FIGS. 1-5, within the sealed interior volume 58 of mold assembly 60, the exothermic foam-forming resin 66 expands and pressurizes interior volume 58 of mold assembly 60 consolidating the fabric layers of door skin 50 and simultaneously heating the resin to above the cure temperature. Once the resin in the fabric layers of door skin 50 is fully cured, mold assembly 60 is opened and bladder 64 containing the now-cured exothermic foam-forming resin 66 is removed together with the completed door skin 50. Bladder 64 together with the cured exothermic foam-forming resin 66 is then separated from door skin 50 and discarded leaving the door skin 50 complete and ready to use.

Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention should be limited only to extent required by the appended claims and the rules and principals of applicable law. 

1. A method of manufacturing a composite panel having a first side and a second side comprising: providing a first mold half having an interior surface defining the surface configuration of said first side of said composite panel; applying at least one fabric layer to said interior surface of said first mold half, said at least one fabric layer comprising a plurality of fibers coated with a heat curable resin; providing a second mold half; joining said first mold half and second mold half together to provide a substantially sealed interior volume; inserting a bladder into said interior volume in contact with said at least one fabric layer; injecting a catalyzed self-expanding exothermic foam material into said bladder in an amount sufficient to pressurize said interior volume to a predetermined pressure and to simultaneously heat said heat curable resin to a predetermined temperature; allowing said heat curable resin to cure to form said composite panel; and separating said foam material from said cured composite panel.
 2. The method of claim 1 further comprising: applying a non-stick layer between said bladder and said at least one fabric layer.
 3. The method of claim 2, wherein: said non-stick layer comprises a woven glass fabric impregnated with a tetrafluoroethylene polymer.
 4. The method of claim 2 further comprising: allowing said catalyzed self-expanding exothermic foam material to cure into a rigid volumetric body and thereafter detaching said rigid volumetric body from said at least one fabric layer.
 5. The method of claim 1 further comprising: applying a substantially non-expanding exothermic gel coat layer between said at least one fabric layer and said interior surface of said first mold half.
 6. The method of claim 1 further comprising: Applying a substantially non-expanding exothermic resin between said at least one fabric layer and said bladder.
 7. The method of claim 1, wherein: said heat curable resin is applied to said plurality of fibers of said at least one fabric layer before said at least one fabric layer is applied to said interior surface of said first mold half.
 8. The method of claim 1, wherein: said heat curable resin is applied to said plurality of fibers of said at least one fabric layer after said at least one fabric layer is applied to said interior surface of said first mold half.
 9. The method of claim 1, wherein: said interior surface defining the surface configuration of said first side of said composite panel comprises a surface defining a residential door skin.
 10. The method of claim 1, wherein: said interior surface defining the surface configuration of said first side of said composite panel comprises a surface defining a leg brace.
 11. The method of claim 1, wherein: said catalyzed self-expanding exothermic foam material has an exothermic rate sufficient to maintain said heat curable resin at a temperature of least 150 degrees Fahrenheit for a minimum of 30 minutes.
 12. The method of claim 1, wherein: said composite panel comprises an orthopedic brace.
 13. The method of claim 1, wherein: said composite panel comprises a residential door skin. 